I think you and your husband are focused on two different things here.
For you, it's the animal welfare issue. Which, many of us have acknowledged, is indeed a worthy thing to be concerned about.
For your husband, however, it is reliving a moment he enjoyed as a child and wants to share with his children.
Of course, they are your children too. And that's where the hard part comes in-- accepting that the two of you have differing priorities.
There is something grand and magical about seeing a traditional circus as a little kid. I remember going. I also remember taking a little boy I used to nanny, going as an adult, and feeling very aware of the fact that this wasn't the life the animals were supposed to have. And feeling bad.It's clear your husband hasn't (yet) had that experience.
So, just something to think about-- he's not wanting to take the kids to the circus to watch animals being abused-- your husband is wanting to recreate something which he loved as a kid. So, try to keep that in mind. If it were my husband and I in this situation, to be honest, I would probably let him take our son, because of what it would mean 'for them'. Because, sometimes love is really illogical. It's probably a lone opinion. I don't support factory farming, don't eat land animals... but I would let my husband take my son if it was really meaning a lot to him. Because I might want to be treated the same way if there were something which meant a lot to me to do with our son, too. They might go and find it upsetting, like some posters have experienced-- but I think that sometimes, it's really important not to let *my* values dictate everything that happens in the relationship. It's a group work.